Flux of CO2 from the Lithosphere in the Bohemian Massif

Abstract

THE important feature of the NW part of the Variscan Bohemian Massif in Czechoslovakia1,2 is a 140 km long graben structure that was tectonically active in the Oligocene and Miocene. Tertiary and early Quaternary volcanic activity and present day hot and cold mineral springs, of maximum temperature 72° C, spatially coincide with the structure. Some of the springs produce gaseous CO2. The rocks yielding the water are felsic igneous or felsic metamorphic rocks. The recent study of natural equilibration between meteoric water and felsic rocks in the area3 indicates that the hydrolysis of primary aluminium silicates and the formation of clay minerals and calcite are the chief processes which control the chemistry of the subsurface waters. Because limestones and dolostones which can release enormous quantities of CO2 by natural thermal decomposition4 are scarce in the region and abundant amounts of organic matter are not likely to be present in the felsic rocks the large amount of CO2 in the subsurface waters must be derived from other sources. It has been widely assumed that the gaseous CO2 in the springs represents a final stage of the Tertiary and early Quaternary volcanic activity5. According to the available geological evidence6 it may be concluded that both the CO2 and the basaltic volcanism are manifestations of deep seated fault systems along which CO2 of a deep, not well understood, origin escapes from or through the lithosphere. If this is true, higher partial pressures of CO2 should be encountered in the subsurface waters of the NW part of the Bohemian Massif, as compared to localities where no excessive flux of CO2 is reported.